A gating ruler (also known as grating ruler displacement sensor or grating ruler sensor) is a measurement feedback device which operates using the optical principles of grating; the grating ruler may serve to detect a linear displacement or an angular displacement, and it is typically used in closed loop servo-system for numerically controlled machine tool.
At present, the grating rulers are mainly divided into three general categories, the incremental grating ruler, the absolute grating ruler, and the incremental grating with distance code reference marks. The position detecting signals of the grating ruler are divided into two categories, the incremental count output and the absolute value output; for the former, it is required to calculate the numbers of the output impulses to determine the actual position, and thus it is referred to as incremental grating ruler; the output signal of the latter may directly reflect the actual position, and thus it is referred to as absolute grating ruler.
As the processing industries become more precise, the role of the grating ruler in the field of processing feedback is more and more significant; meanwhile, in order to increase the processing precision, to reduce cumulative errors and to avoid complicated steps of startup and positioning, etc., the advantages of the absolute grating ruler become more and more obvious.
Although the absolute grating ruler has significant advantages over the incremental grating ruler, its implementation requires a reliable mechanic structure, and its requirements for mounting process are stricter than the incremental grating ruler. In particular for CMOS/CCD sensor-based absolute grating ruler, relatively high requirements are imposed on the optical depth of field, the straightness of the grating ruler body, and the parallelism between the CMOS sensor and the ruler body. If there is an angle between the mounted ruler body and the CMOS, then decoding errors may occur, and if the straightness of the ruler body is not adequate, then a cumulative error will occur. The present invention may solve this problem.